Hi,

As I experience problem the same with yours, I found that the mechanical construction is the problem(relay switch). Electric current that flow through relay contactor can cause a strange fenomena to the circuit itself.

The reseting problem is not found when I turn my cellphone and put it arround all pinout (very very close). The base frequency at about 900-1800MHz. So, there is no problem I face with EMI to microcontroller.

Solution, just put an isolator between your microncontroller pinout to relay, optoisolator. I'm sure your problem will be solved. Or, take a solid state relay to replace your mechanical relay.